This invention relates to completing subsea wellheads.
After a subsea well is drilled, but prior to the installation of what is commonly referred to as a horizontal or side valve tree type completion system, it is necessary to make the well safe before recovering the drilling Blowout Prevention Subsea Stack (BOP). In order to make the well safe, cement plugs or mechanical sealing plugs must be installed in the bore of the last casing string (production casing) of the wellhead, in order to prevent the possibility of hydrocarbons escaping to the subsea environment after removal of the BOP. After successful installation of the horizontal/side valve tree onto the wellhead, it is necessary to remove the downhole casing cement plug/s or mechanically set casing sealing plug/s. Cements plugs are removed by drilling out with standard downhole drill bits. Of course, these plug recovery operations require passing drill bits or special plug recovery tools through the side valve tree and could expose critical internal through bore profile features in the side valve tree to the following ramifications:
a. Mechanical damage as drill bit or special mechanical plug recovery tools pass through the side valve tree. PA1 b. Mechanical damage due to drill pipe rotation during drilling. PA1 c. Exposure of annulus, production, and control line side outlet penetrations to drilling muds and drilling returns which could clog up and impair the operation of the side valve tree critical side outlet valve and control functions. Additionally, drilling muds and returns might accumulate on the surface of the through bore profile and be very difficult to remove, which would in turn potentially prevent the successful installation of internal tubing hanger and/or tree cap. PA1 d. Finally, it is desirable not to expose the side valve tree side outlet valve and control line bores to any hydraulic pressures resulting from the drilling operation which could result in pumping undesirable fluids into valve or control line porting.
One of the key benefits ascribed to horizontal/side valve tree type completions is the ability to recover or recomplete the well without having first to recover the horizontal/side valve tree. This provides the ability to recover the tubing hanger and downhole completion through the tree. This also provides the ability to perform additional drilling operations through the tree such as side-tracking into a different hydrocarbon formation which allows the same wellhead and tree to be used for recompleting the well from that different geological hydrocarbon formation. Side tracking requires even more extensive drilling through the horizontal/side valve tree with thus even greater potential for detrimental ramifications to the side valve tree internals as described above for drilling out cement plugs, should no type of protection be provided for the tree through bore.
All of the detrimental ramifications to the side valve tree internals described above during downhole/drilling operations can be prevented if a pressure containing tree bore protector is installed in the horizontal/side valve tree prior to performing any of the described downhole drilling operations. Such a bore protector would lockdown to the internal profile in the side valve tree while sealing off above and below critical internal profile features in the side valve tree such as: Annulus side outlets, Production bore side outlets, Control line side outlets, Tubing hanger and tree cap critical internal profile landing, lockdown, and orientation features.
Additionally, the bore protector should be designed to withstand and isolate the tree features from any expected formation or drilling pressures which might be encountered or applied as part of downhole drilling operations, while allowing passage of a drill bit through same. The internal bore diameter of the bore protector would be of a size equal to or greater than the inside diameter of the last casing string/production casing.